The 3rd-Generation Partnership Project (3GPP) has developed a wireless communication technology known as Long Term Evolution (LTE), as documented in the specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). LTE is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNodeBs or eNBs in 3GPP documentation) to user terminals (referred to as user equipment, or UEs, in 3GPP documentation) are sent using orthogonal frequency-division multiplexing (OFDM). OFDM splits the transmitted signal into multiple parallel sub-carriers in frequency.
More specifically, LTE uses OFDM in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can be viewed as a time-frequency resource grid. FIG. 1 illustrates a portion of the available spectrum of an exemplary OFDM time-frequency resource grid 50 for LTE. Generally speaking, the time-frequency resource grid 50 is divided into one millisecond subframes. As shown in FIG. 3, each subframe includes a number of OFDM symbols. For a normal cyclic prefix (CP) length, which is suitable for use in situations where multipath dispersion is not expected to be extremely severe, a subframe consists of fourteen OFDM symbols. A subframe has only twelve OFDM symbols if an extended cyclic prefix is used. In the frequency domain, the physical resources are divided into adjacent subcarriers with a spacing of 15 kHz. The number of subcarriers varies according to the allocated system bandwidth. The smallest element of the time-frequency resource grid 50 is a resource element. A resource element consists of one OFDM subcarrier during one OFDM symbol interval.
LTE resource elements are grouped into resource blocks (RBs), each of which, in its most common configuration, consists of twelve subcarriers and seven OFDM symbols (one slot). Thus, a RB typically consists of 84 resource elements. The two RBs occupying the same set of twelve subcarriers in a given radio subframe (two slots) are referred to as an RB pair, which includes 168 resource elements if a normal CP is used. Thus, an LTE radio subframe is composed of multiple RB pairs in frequency, with the number of RB pairs determining the bandwidth of the signal.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. This is shown in FIG. 2.
The signal transmitted by an eNB to one or more UEs may be transmitted from multiple antennas. Likewise, the signal may be received at a UE that has multiple antennas. The radio channel between the eNB distorts the signals transmitted from the multiple antenna ports. To successfully demodulate downlink transmissions, the UE relies on reference symbols (RS) that are transmitted on the downlink. Several of these reference symbols are illustrated in the resource grid 50 shown in FIG. 3. These reference symbols and their position in the time-frequency resource grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages include commands to control functions such as the transmitted power from a UE, signaling to identify RBs within which data is to be received by the UE or transmitted from the UE, and so on.
Specific allocations of time-frequency resources in the LTE signal to system functions are referred to as physical channels. For example, the physical downlink control channel (PDCCH) is a physical channel used to carry scheduling information and power control messages. The physical HARQ indicator channel (PHICH) carries ACK/NACK in response to a previous uplink transmission, and the physical broadcast channel (PBCH) carries system information. The primary and secondary synchronization signals (PSS/SSS) can also be seen as control signals, and have fixed locations and periodicity in time and frequency so that UEs that initially access the network can find them and synchronize. Similarly, the PBCH has a fixed location relative to the primary and secondary synchronization signals (PSS/SSS). The UE can thus receive the system information transmitted in BCH and use that system information to locate and demodulate/decode the PDCCH, which carries control information specific to the UE.
As of Release 10 of the LTE specifications, all control messages to UEs are demodulated using channel estimates derived from the common reference signals (CRS). This allows the control messages to have a cell-wide coverage, to reach all UEs in the cell without the eNB having any particular knowledge about the UEs' positions. Exceptions to this general approach are the PSS and SSS, which are stand-alone signals and do not require reception of CRS before demodulation. The first one to four OFDM symbols of the subframe are reserved to carry such control information. The example subframe shown in FIG. 3 has a control region of three OFDM symbols. The actual number of OFDM symbols reserved to the control region may vary, depending on the configuration of each cell. The particular number n=1, 2, 3 or 4 for a given cell is known as the Control Format Indicator (CFI), and is indicated by the physical CFI channel (PCHICH), which is transmitted in the first symbol of the control region.
Downlink transmissions in LTE are dynamically scheduled, meaning that in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, for the current downlink subframe. The dynamic scheduling information is communicated to the user equipments (UEs) via the PDCCH, which is transmitted in the control region. After successful decoding of a PDCCH, the UE performs reception of the Physical Downlink Shared Channel (PDSCH) or transmission of the Physical Uplink Shared Channel (PUSCH) according to pre-determined timing specified in the LTE specs. In addition to the PDCCH, the control region in the downlink signal from the base station also contains the Physical HARQ Indication Channels (PHICH), which carry hybrid-ARQ acknowledgements (ACK/NACK) corresponding to uplink transmissions from the UEs served by the base station.
The downlink Layer 1/Layer 2 (L1/L2) control signaling transmitted in the control region thus consists of the following different physical-channel types:                The Physical Control Format Indicator Channel (PCFICH), informing the terminal about the size of the control region (one, two, or three OFDM symbols). There is one and only one PCFICH on each component carrier or, equivalently, in each cell. The Physical Downlink Control Channel (PDCCH), used to signal downlink scheduling assignments and uplink scheduling grants. Each PDCCH typically carries signaling for a single terminal, but can also be used to address a group of terminals. Multiple PDCCHs can exist in each cell.        The Physical Hybrid-ARQ Indicator Channel (PHICH), used to signal hybrid-ARQ acknowledgements in response to uplink UL-SCH transmissions. Multiple PHICHs can exist in each cell.        
The PDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI includes:                Downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the PUCCH used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH carries one DCI message with one of the formats above. Since multiple terminals can be scheduled simultaneously, on both downlink and uplink, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on a separate PDCCH, and consequently there are typically multiple simultaneous PDCCH transmissions within each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH is selected to match the radio-channel conditions.
Control messages can be categorized into messages that need to be sent only to one UE (UE-specific control) and those that need to be sent to all UEs or some subset of UEs (common control) within the cell being covered by the eNB. Messages of the first type, UE-specific control messages, are typically sent using the PDCCH.
Control messages of PDCCH type are demodulated using CRS and transmitted in multiples of units called control channel elements (CCEs) where each CCE contains 36 REs. A PDCCH message may have an aggregation level (AL) of one, two, four, or eight CCEs. This allows for link adaptation of the control message. Each CCE is mapped to nine resource element groups (REGs) consisting of four RE each. The REGs for a given CCE are distributed over the system bandwidth to provide frequency diversity for a CCE. This is illustrated in FIG. 4. Hence, a PDCCH message can consist of up to eight CCEs, spanning the entire system bandwidth in the first one to four OFDM symbols, depending on the configuration.
The LTE Paging Procedure
In LTE networks, a UE is in a RRC_CONNECTED mode or state when a Radio Resource Control (RRC) connection has been established between the UE and the network. Otherwise, the UE is in an RRC_IDLE mode or state. The LTE network uses a paging process to initiate access to a terminal when the UE is in RRC_IDLE mode. Details corresponding to a paging message are scheduled with a DCI message in the common search space, with the Cyclic Redundancy Check (CRC) field of the DCI message scrambled with a P-RNTI. The DCI message points to a corresponding message that is sent on PDSCH. For the purposes of this disclosure, the term “paging message” refers to any control channel message that alerts the UE to the existence of a page. The data carried by the PDSCH and pointed to by the paging message is referred to herein as the “paging message details.” In LTE systems, the “paging message” and the “paging message details” can thus be viewed as separate messages. In other systems, however, a “paging message” may carry the paging message details itself.
When the UE is in RRC_IDLE mode, the cell in which the UE is located is generally not known by the network. Therefore the paging message is typically transmitted in each of several cells. These several cells form an entity that is called a tracking area. The tracking area is controlled by the Mobility Management Entity (MME), which keeps track of which tracking area the UE belongs to. The MME is able to do this since the UE reports to the MME whenever it enters a new tracking area.
Paging messages targeted to a given terminal are scheduled for transmission in scheduling occasions that occur in a very sparse manner in time. This approach allows the terminal to be in Discontinuous Receive (DRX) state as much as possible, to save battery power. The subframe in which the terminal wakes up and monitors paging messages is given by a formula that takes into account the identity of the terminal, a cell-specific paging cycle and, optionally, a UE-specific paging cycle.
PDCCH Monitoring
LTE defines so-called search spaces, which define the set of CCEs the terminal is supposed to monitor for scheduling assignments/grants relating to a certain component carrier. A search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to decode. As there are multiple aggregation levels, corresponding to one, two, four, and eight CCEs, a terminal has multiple search spaces. In each subframe, the terminals will attempt to decode all the PDCCHs that can be formed from the CCEs in each of its search spaces. If the Cyclic Redundancy Check (CRC) checks, the content of the control channel is declared as valid for this terminal and the terminal processes the information (scheduling assignment, scheduling grants, etc.). Each terminal in the system therefore has a terminal-specific search space at each aggregation level. These terminal-specific search spaces are collectively called the UE-specific search space (USS).
In several situations, there is a need to address a group of, or all, terminals in the system. To allow all terminals to be addressed at the same time, LTE has defined common search spaces in addition to the terminal-specific search spaces. Again, while there is a common search space for each aggregation level, these are often collectively referred to as the common search space (CSS). The common search space is, as the name implies, common, and all terminals in the cell monitor the CCEs in the common search spaces for control information. Although the motivation for the common search space is primarily transmission of various system messages, it can be used to schedule individual terminals as well. Thus, it can be used to resolve situations where scheduling of one terminal is blocked due to lack of available resources in the terminal-specific search space. More importantly, the common search space is not dependent on UE configuration status. Therefore, the common search space can be used when the network needs to communicate with the UE during UE reconfiguration periods.
A UE thus monitors a common search space and a UE-specific search space in the PDCCH. In each of these search spaces, a limited number of candidates (equivalently, PDCCH transmission hypotheses) are checked, in every downlink subframe for which the UE is in RRC_CONNECTED mode and in a non-DRX interval. For a UE in RRC_IDLE mode, the UE monitors the common search space at least for each paging subframe that is part of the paging cycle. These hypotheses are known as blind decodes, and the UE checks whether any of the transmitted DCI messages is intended for it. The UE knows that the downlink control information is intended for it if the scrambling mask of the CRC of the control message is identical to the expected RNTI of the message. For instance, if a paging message is expected in a subframe, the UE searches the transmitted control channels in that subframe for a message with CRC scrambled with the paging-RNTI (P-RNTI). The UE also monitors other RNTI, such as C-RNTI for scheduling of the shared data channel or the SI-RNTI for scheduling of system information.
Enhanced Control Signaling
As of Release 11 of the standards for LTE, it has been agreed to introduce UE-specific transmission for control information in form of enhanced control channels by allowing the transmission of generic control messages to a UE using UE-specific reference signals and the data region. This enhancement means that precoding gains can be achieved also for the control channels. Another obvious benefit is that more resources may be used for control signaling, as needed.
Extended Coverage
In a future “networked society” scenario it is expected that there will be a very large number of machine-type-communication (MTC) devices active in wireless networks. Many such devices will transmit small amount of uplink data, e.g., 100 bits or so, very occasionally, e.g., once per hour or so. In the 3GPP working groups standardizing improvements to LTE systems, there are plans to introduce a new solution for “enhanced MTC coverage,” with a target to improve the link budget by approximately 15-20 dB compared to the link budgets supported by the legacy LTE standard. This will make LTE even more attractive for the deployment of MTC devices and applications. Note that the terms “extended coverage” and “enhanced coverage” are used interchangeably, and refer to technical solutions, in a wireless network, that provide substantially improved link budgets, compared to normal communications in the wireless network, typically by providing an alternative communication scheme that employs a much higher degree of redundancy than normal communications in that same wireless network.
To achieve coverage enhancements on the order of 15-20 dB in LTE, multiple physical channels and physical signals will need to be improved. The required improvements are quite large—20 dB coverage improvement is equivalent to operation at 100 times lower signal-to-noise ratios—and LTE is already very good, in that there is no known flaw in the current design of LTE that can be corrected to provide improvements anywhere near 100 times. As a result, it is likely that most of the required coverage improvements will be achieved through the transmitting of highly redundant information, e.g., through ordinary repetition of the transmitted information. Current LTE signals cannot easily just be repeated approximately 100 times, for example due to timing constraints during connection setup and other procedures, so new signals and related procedures will likely need to be defined for this purpose.